I have been using Trello to keep my different lists organized: Inbox, Today, This Week, Waiting For, and Done. Then I use labels to determine priority: Important, Not Important, Urgent and Not Urgent.

Here is a great article by Gautam Ramdurai, who works at Google on how to incorporate Trello with your GTD workflow: https://medium.com/precise-curiosity/locus-focus-an-approach...

Todoist is hands down my favorite. It has integration with lots of other software (Gmail, Outlook for Windows, Postbox, Chrome, etc..). Additionally it has convenient quick keys for adding a task to your inbox without having to switch applications. I find their karma system enticing. I have no affiliation with the company, just a huge fan of their work. I was on a Wanderlust kick for a while, but their reliability has been troubling.

I needed something that lets me organize tasks and notes in a tree like manner, like Workflowy, but with proper documents and more text editing functionality, like Evernote, so I built it myself: https://flownote.io

OmniFocus with Airmail and Slack. True, everything Omni is expensive but the sheer impact is has on my life is priceless. I found every other GTD app too limited.

Things by Cultured Code. Mac and iOS only, though. But much cheaper and more simple than Omnifocus.